Lion vs. the Ant: An Ecological Story

The science of ecology (from Greek oikos=house and logos=science), as defined in the famous Encyclopaedia Britannica, can be explained as “the branch of science that studies the relationship between groups of living things and their environment”. Ecosystems, which are based on highly delicate balances, are not systems that fall apart when one link is broken, but rather networks that establish new connections and find new dynamics by changing when one element is damaged. A recent change in a Kenyan nature reserve provides an interesting, enlightening, and alarming example of how dynamics can shift (often resulting from human activities).

Let us first describe the scene in Kenya in terms of the ecosystem and the relationships between its inhabitants.

The whistling thorn (Vachellia drepanolobium) is a tree species native to the savannahs of East Africa (herbaceous grasslands across the Equator) and is named after the sounds it makes in the wind. It is an actor of symbiosis that we may encounter in many ecosystems. This acacia species is home to four different species of ants that live among its ball-shaped spines and feed on the tree’s nectar. These ants protect the tree (and their own food supply) by attacking mega herbivores such as elephants, giraffes, and rhinos, which come to feed on the tree’s delicious leaves. This ensures that the resilient, strong, and fertile acacia trees of the savannah continue to be home to ants, as well as many species of birds and a variety of other creatures.

An invasive species of ant (Pheidole megacephala), known as the “big-headed ant,” arrived in Kenya from South Africa about 15 years ago and began to disturb this order. The aggressive foreign guest that preys on many animals from insects to small birds was certainly a species that the acacia ants would not want to encounter. However, it did reach the acacia trees, killing adult acacia ants and eating both their young and their eggs, leaving the acacia vulnerable to herbivores. According to scientists, trees that are no longer protected by ants are five to seven times more likely to be consumed by elephants.

Once the problem became significant, researchers, curious about the extent of the threat posed by these big-headed ants, considered to be one of the worst invasive species ever, began surveying the Laikipia region in Kenya. They divided the study area into two and identified two separate regions that were invaded and uninvaded by the ants. Then, they divided these areas into separate sections and built fences around some to block the entrance of large herbivores. Their aim was to observe and assess the effect of ants and herbivores both separately and together. The research lasted three years.

In ant-infested areas without fences, arboreal cover was reduced to one-third because elephants knocked down the trees to eat their leaves. In other ant-infested areas where elephant entrance was fenced off, most of the trees were unharmed. This also had repercussions in the field of visibility through the woods.

To figure out how other animals would be affected by this, the researchers began tracking a group of six lionesses using GPS tags. Since lions hunt by hiding and ambushing, could the damage to these trees have affected the lion population? Indeed, visibility in the ant-infested areas had increased by 13 meters, and the lions’ successful zebra hunts had dropped to a third. This raised a new question: if lions were now hunting less, why weren’t their numbers declining? The answer: Adaptation. As the lions were no longer able to hunt enough zebras, they had turned their attention to the African buffalo. Buffaloes now made up 42% of their prey, but this also required them to hunt in larger groups as buffaloes are bigger and more competitive than zebras.

The researchers also acknowledge the shortcomings of their study. For example, even though the lion populations have not yet declined, one cannot say they never will; buffaloes are risky prey. Also, if the number of trees continues to decline, it may become more difficult for lions to hunt buffaloes too, and the dynamics may shift even further.

The fact that such a small creature can have such an impact on the entire ecosystem shows how delicate the ecosystems dynamics are. The ants have not only caused the destruction of trees, but they also seem to have changed the behaviour and diet of lions, destroyed bird habitats, and possibly started to degrade the soil health of the area. Acacia trees are nitrogen fixators thanks to the bacteria living in their roots, thus benefiting other plants growing around them. Additionally, there are other worrisome possibilities including negative impacts on butterflies that interact with acacia ants and on the endangered black rhinos that feed on acacia leaves.

“It’s kind of depressing,” says Todd Palmer, one of the study authors and a biology professor at the University of Florida, “When most people think about conservation, they think about conserving species–saving the elephants or the lions or the zebras. We think a lot less often about the integrity of species interaction.” A lion isn’t just a lion, it’s part of a biological fabric. To keep that living cloth intact, every thread plays a role, even the humble ant.